This invention pertains to ethylene homopolymerization and copolymerization catalysts and more particularly to a method for preparing supported chromium polymerization catalysts.
Ethylene polymerization catalysts containing chromium and titanium are old in the art. These catalysts were an improvement over prior catalysts containing chromium alone in that they afforded lower molecular weight ethylene polymers which can be processed more easily than higher molecular weight polyethylenes. In particular several patents have described specific methods for producing titanium modified chromium catalysts for ethylene polymerization. U.S. Pat. No. 3,349,067 discloses a process of preparing a polymerization catalyst which comprises contacting a carrier material with a solution of chromium-containing compound in anhydrous organic solvent. In this disclosure the specifications describe reaction in the presence of silica of chromyl chloride (CrO.sub.2 Cl.sub.2) and tetra (n-butyl) titanate to form a precipitate, containing titanium and chromium, on the silica support. Such a process therefore depends on the reaction product of chromyl chloride and the organic titanate to deposit, in no unique way, on the substrate.
U.S. Pat. No. 3,622,521 describes a catalyst formed by first adding to a support a chromium-containing compound using an aqueous medium and then adding a titanium-containing compound using a non-aqueous medium. Such a process involves the use of two different solvents involving two separate and distinct deposition steps. Such a process can be cumbersome and lead to non-reproducible catalysts for ethylene polymerization.
U.S. Pat. No. 3,879,362 deals with a process for polymerizing ethylene using a catalyst prepared by (1) heating together a titanium compound and a support material selected from silica, alumina, zirconia, thoria or composites thereof and (2) heating the product from (1) with a chromium compound at a temperature in the range 350.degree.-550.degree. C. to produce an active catalyst. A number of chromium compounds is cited in the specifications including bis(cyclopentadienyl) chromium, i.e., chromocene. The main attention is devoted to this two step process involving either chromium oxide or tertiary butyl chromate as the source of the chromium compound.
All of the above cited routes for preparation of these modified catalysts can cause problems. The catalysts are sensitive to the methods of preparation thereby producing polyethylenes which vary in polymer properties such as molecular weight, molecular weight distribution, and density. Catalyst activity also varies depending on the method of catalyst preparation.
It is therefore an object of this invention to provide a catalyst for ethylene polymerization which is simple to prepare and which exhibits consistent catalytic activity.
Other objects will be apparent to those skilled in the art upon a reading of the specification.